feat(gossip-support) handle active leaves

[EclesioMeloJunior]

Description

• Given the current values in the state, last failure and last connection request, the subsystem should force new requests or re-resolve authorities
• Determine if the current session index has changed, if so determine relevant validators and issue connection request. The subsystem should change the last sesison index value only if it is possible to retrieve the new session info.
  • Don't need to implement update_gossip_topology, get_key_index_and_update_metrics (handled by other issues)
• If we notice that a new session is starting we should update our authority ids cache and send a NetworkBridgeRxMessage::UpdatedAuthorityIds message with informations about the new set of authorities.

Links

https://github.com/paritytech/polkadot-sdk/blob/master/polkadot/node/network/gossip-support/src/lib.rs

Acceptance Criteria

• Unit test for the handler